The present invention relates to a new design of the foreship of a vessel of the displacement type. More specifically, the invention relates to an arrangement as disclosed in the preamble of independent claim 1.
Ever since the start of oil activities in the North Sea, all vessels engaged in marine operations have been characterised by having their wheelhouse and superstructure with equipment located immediately behind the forecastle deck and extending right up to the collision bulkhead. A large loading deck or working deck has been located aft of the superstructure and wheelhouse. This arrangement is a legacy of the first platform supply vessels which were used on the British continental shelf in the 1950s and is still the dominant solution today.
When these vessels are not in operation or are waiting on the weather (backing), they will normally have their bow turned upwind and into the waves.
The bow design has been a raked bow or straight stem line up to the design water line and above that a raked bow, or a solution including a bulb and a raked bow where the bulb can be defined as a bulge in the hull, primarily below the water line, to improve the wave system and resistance.
Typical negative effects of the aforementioned conventional bow forms are that they reflect incoming waves to too great a degree (wave formation gives energy loss), they are heavily subjected to the slamming of waves against the ship side, they generate a great deal of spray and they are subjected to the occurrence of green water on the deck in front of the superstructure.
When this type of bow shape meets incoming waves, the submerged volume (later referred to as “facing volume”) increases very quickly, buoyancy increases as quickly and the retardation force becomes very large. These effects intensify with increasing wave height and increasing speed of the vessel into the wave direction.
Vessels with their superstructure and wheelhouse located on the weather deck immediately aft of the collision bulkhead and having a hull shape as described above are highly vulnerable to heavy weather damage.
The maximum speed at sea for these vessels is determined primarily by the water line length, entry angles of the foreship and the proportion of outwardly sloping frame lines in the bow, and by available thrust forces from the propeller(s). Typical maximum speed in calm water is about 13-16 knots, and they will normally have a loss of speed of about 3 to 5 knots in head sea. The speed loss in head sea is a direct result of the energy loss due to reflected waves, which results in retardation of the vessel.
Both during headway, in particular in head sea, and during backing/waiting, the crew, who have their quarters forward, will experience large accelerations and retardations. Such motions reduce the quality of their rest periods and have an adverse effect on their capacity to work. Reduced resting time and difficult working conditions increase the risk of accidents and injuries.
To reduce or eliminate the aforementioned drawbacks of the prior art, there is provided, according to the present invention, an arrangement as disclosed in the characterising clause of claim 1.
Advantageous embodiments of the invention are set forth in the dependent claims.